Backside configured surface plasmonic structure for infrared photodetector and imaging focal plane array enhancement

ABSTRACT

The invention relates to quantum dot and photodetector technology, and more particularly, to quantum dot infrared photodetectors (QDIPs) and focal plane array. The invention further relates to devices and methods for the enhancement of the photocurrent of quantum dot infrared photodetectors in focal plane arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 61/806,098, filed on. Mar. 28, 2013, which isincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to quantum dot and photodetector technology, andmore particularly, to quantum dot infrared photodetectors (QDIPs) andfocal plane array. The invention further relates to devices and methodsfor the enhancement of the photocurrent of quantum dot infraredphotodetectors in focal plane arrays.

BACKGROUND OF THE INVENTION

Quantum dot infrared photodetectors (QDIPs) based on intersubbandtransitions in self-assembled InAs quantum dots (QDs) have beenextensively researched for middlewave infrared (MWIR, 3-5 μm) andlongwave infrared (LWIR, 8-12 μm) photodetection and imaging [1-8]. Thethree-dimensional (3D) quantum confinement structure providesadvantages, such as normal incidence photodetection [1, 2], lower darkcurrent [5], high photoreponsitivities [8], and high operatingtemperature [3-5, 8]. In fact, so far, only the QDIP technology has beenreported to have a high operating temperature of over 298 K [5, 9-11].The major issue of the QDIP technology, however, is that the totalnumber of QD layers that can be stacked in a QDIP is limited by theaccumulation of strain and the strain induced defects and dislocations.This leads to a thin active QD absorption region, which results in a lowpercentage of light that can be absorbed in the active region. What iscurrently needed are techniques for employing QDIP technology in focalplane array applications wherein the QDIP has increased detectivity tomatch and exceed the performance of current IR detectors.

SUMMARY OF THE INVENTION

The invention relates to quantum dot and photodetector technology, andmore particularly, to a quantum dot infrared photodetector and focalplane array. The invention further relates to devices and methods forthe enhancement of the photocurrent quantum dot infrared photodetectorsin focal plane array.

In one embodiment, the present invention contemplates focal plane arraydevice containing backside configured surface plasmon polaritons (SPP)structures. In one embodiment, said. SPP structure is within the quantumdot infrared photodetector. In one embodiment, said SPP structurecomprises plasmonics near-field effect. In one embodiment, the SPPstructure is configured on the backside of the QDIP. In one embodiment,the plasmonics near-field effect comprises an enhanced photocurrent. Inone embodiment, a passivation layer is deposited on top of the SPPstructure further comprises a passivation layer. In one embodiment, thepassivation layer comprises an enhanced near-field effect resonance. Inone embodiment, the passivation layer comprises a reduced FPA surfacedark current.

In one embodiment, the invention relates to a backside configured focalplane array device comprising: (a) a semi-insulating substrate; (b) afirst contacting layer in direct and continuous contact with saidsemi-insulating substrate; (c) a set of first metal contacts in directcontact with said first contacting layer; (d) a second contacting layer;(e) a series of quantum dot layers disposed between said firstcontacting layer and said second contacting layer; (e) surface plasmonpolaritons structures in direct contact with said second contactinglayer; (f) a silicon nitride (SiN_(x)) passivation layer deposited oversaid surface plasmon polaritons structures; (g) a second metal contactin contact with said second contacting layer; and (h) an indium bump incontact with said a SiN_(x) passivation layer deposited over saidsurface plasmon polaritons structures and said second metal contact. Inone embodiment, said semi-insulating substrate comprises GaAs. In oneembodiment, the SiN_(x) passivation layer upon the backside of saidseries of quantum dot layers is also between said first contacting layerand said second contacting layer, said second contacting layer, and saidsecond metal contact in contact with said second contacting layer andcovers the sidewall of the photodetector. In one embodiment, the quantumdot layers are comprised of gallium, indium, aluminum and arsenic. Inone embodiment, the device is capable of normal incidence detection. Inone embodiment, there is a buffer layer between said first contactinglayer and said series of quantum dot layers. In one embodiment, there isa buffer layer between said second contacting layer and said series ofquantum dot layers.

In one embodiment, the invention contemplates a backside configuredfocal plane array device comprising: (a) a series of quantum dot layersdisposed between a first contacting layer and a second contacting layer;(b) back side configured surface plasmon polaritons (SPP) structures indirect contact with said second contacting layer; (c) a SiN_(x)passivation layer deposited over said surface plasmon polaritonsstructures; (d) a metal contact in contact with said second contactinglayer; and (e) an indium bump in contact with said a silicon nitridepassivation layer deposited over said surface plasmon polaritonsstructures and said metal contact.

In one embodiment, the invention contemplates a method of enhancing thephotocurrent spectrum response of a quantum dot infrared photodetectorby at least four times comprising: (a) increasing the photocurrent withan surface plasmon structure integrated into said photodetector, and (b)decreasing the dark current with a silicon nitride passivation layer. Inone embodiment, the photocurrent is enhanced by at least sixteen times.In one embodiment, the quantum dot infrared photodetector is part of afocal plane array.

In one embodiment, the invention contemplates a backside configuredquantum dot infrared focal plane array device comprising an array ofpixels. In one embodiment, each pixel of said focal plane array has aseries of quantum dot layers disposed between a first contacting layerand a second contacting layer; a metal contact in contact with saidsecond contacting layer; and an indium bump in contact with said metalcontact; wherein the improvement comprises: a) surface plasmonpolaritons structure in contact a contacting layer, and b) a passivationlayer which covers said surface plasmon polariton structure and is incontact with said indium bump and metal contact and covers the sidewallof the contacting layers, buffer layers, and QDIP layer, and (c)fabricating the surface polariton structures is the first step in thefocal plane array fabrication process.

In one embodiment, the invention contemplates a backside configuredplasmonic polariton enhanced infrared photodetector and focal planearray devices having a series of quantum dot layers disposed between afirst contacting layer and a second contacting layer; a metal contact incontact with said second contacting layer; and an indium bump in contactwith said metal contact; wherein the improvement comprises: a) surfaceplasmon polaritons structure in contact a contacting layer, and b) apassivation layer which covers said surface plasmon polaritionsstructure and is in contact with said indium bump and metal contact, andc) a passivation layer which covers the sidewall of the contactinglayers, buffer layers, and QDIP layer. In one embodiment, the devicefurther comprises d) wherein the surface plasmonic polariton structureis configured on the opposite side of the substrate, and the lightincident is upon on the substrate side (front side). In one embodiment,the device further comprises e) wherein the surface plasmonic polaritonstructures are first fabricated before the fabrication of the pixels ofthe FPA.

In one embodiment, the invention contemplates a method of producing afocal plane array comprising, a) formation of a two dimensional holearray (2DHA) plasmonic structure upon a QDIP structure usingphotolithography, followed by deposition of metal (75 (best) 30 nm gold)to form a mesa, b) patterning of said mesa using photolithography,followed by wet etch stopping at the bottom contact layer of said QDIPstructure, c) formation of ohmic contacts, d) deposition of apassivation layer, e) opening of a connection to the ohmic contactthrough passivation layer, f) patterning and deposition of an indiumbump, and h) hybridization of said indium bumps to a read outintegration circuit.

Even though the experimental data are based on QD infrared detectors, inone embodiment, the invention can be used for any kind of detectors andFPAs for performance enhancement.

DEFINITIONS

To facilitate the understanding of this invention, a number of terms aredefined below. Terms defined herein have meanings as commonly understoodby a person of ordinary skill in the areas relevant to the presentinvention. Terms such as “a”, “an” and “the” are not intended to referto only a singular entity, but include the general class of which aspecific example may be used for illustration. The terminology herein isused to describe specific embodiments of the invention, but their usagedoes not delimit the invention, except as outlined in the claims.

As used herein, the term “backside configured” refers to a device isconfigured such that the surface plasmonic polariton structure isconfigured on the opposite side of the substrate, and the light incidentis upon on the substrate side, which is referred to as the “front side”.

As used herein, the term “quantum dot infrared photodetector” refers toan infrared photodetector using quantum dots (QD) as the active infraredabsorption material.

As used herein, the term “focal plane array” refers to image sensingdevice consisting of an array (typically square) of light-sensing pixelsat the focal plane.

As used herein, the term “semi-insulating substrate” refers to anundoped GaAs substrate that does not conduct electricity.

As used herein, the term “contacting layer” refers to an n-type dopedGaAs layer on which one puts electrodes of the QDIP.

As used herein, the term “buffer layer” refers to an undoped GaAs layerthat one grow to improve the quality of the material.

As used herein, the term “metal contact” refers to electrode for theQDIPs and FPAs

As used herein, the term “QDIP layer” refers to all the layers of thephotodetectors without the electrodes and the surface plasmonicpolariton structure.

As used herein, the term “surface plasmons polaritons (SPP)” refers tolight induced packets of electrical charges that collectively oscillateat the surfaces of metals. Under specific conditions, light thatradiates on the object (incident light) can excite the surface plasmonpolaritons (SPPs). SPPs are guided along metal-dielectric interfaces.SPPs can provide a significant increase in spatial confinement thatbeyond the diffraction limit and local field intensity.

As used herein, the term “SiN_(x)” refers to the silicon nitridepassivation material

As used herein, the term “indium bump” refers to a block of indium, in apreferred embodiment the block of indium has a dimension of 20micrometer×20 micrometer and height of 10 micrometer.

As used herein, the term “pixel” refers to an individual light sensingelement often within the context of a larger device or a focal planearray.

As used herein, the term “normal incidence detection” refers to theincident of the infrared light is from the substrate side (also referredto as the “front side”). The normal incidence is also refer to as“forward looking”.

DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated into and form a part ofthe specification, illustrate several embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The figures are only for the purpose ofillustrating a preferred embodiment of the invention and are not to beconstrued as limiting the invention.

FIG. 1 shows the standard MBE growth structure cross-section.

FIG. 2 shows a standard QDIP structure cross-section.

FIG. 3 shows a cross-section of one embodiment of the current inventionQDIP structure.

FIG. 4 shows the SPP structure (Top View) in addition to a cross-sectionof the larger structure.

FIG. 5 shows a cross-section of the UML273 MBE Growth Structure.

FIG. 6 shows a topside view of the UML273-SP (SEM@430× magnification).

FIG. 7 shows a topside view of the UML273-SP (SEM@3700× magnification).

FIG. 8 shows a topside view of the UML273-SP (SEM@12000× magnification).

FIG. 9 shows a transmission profile of surface plasmon structure onGaAs.

FIG. 10 shows photocurrent spectrum of QDIP versus QDIP-SP.

FIG. 11 shows the enhancement ratio.

FIG. 12 shows a topside picture of the part (4×3 pixels of the 640×512)plasmonic FPA. The small holes are the plasmonic structures.

FIG. 13 shows an infrared image obtained by the plasmonic FPA. Theworking temperature was increased from 77K to 120K, which is over 40K.This is the highest working temperature obtained so far for the longwaveinfrared FPA.

FIG. 14 shows a side view schematic structure of the back-sideconfigured plasmonic polariton structure enhanced FPA. The plasmonicpolariton structures are configured on the other side of the substrate.The light incident is from the front side of the substrate. Fourindividual pixels are shown in FIG. 14.

DESCRIPTION OF THE INVENTION

The invention relates to quantum dot and photodetector technology, andmore particularly, to a quantum dot infrared photodetectors and focalplane arrays. The invention further relates to devices and methods forthe enhancement of the photocurrent response of quantum dot infraredphotodetectors and focal plane array.

In one embodiment, the present invention contemplates a quantum dotinfrared photodetector (QDIP) focal plane array (FPA) with backsideconfigured Surface Plasmon Polaritons (SPP) structure. SPP structuresare capable of enhanced photocurrent by creating plasmonics near-fieldeffect and create a multispectral sensing system. A passivation layermay be deposited on top of SPP structure in order to enhance theresonance of near-field effect as well as reduce the surface darkcurrent on FPA. This embodiment can enhance the responsivity togetherwith the detectivity of QDIP and FPA and the multispectral detector. Thebackside configured plasmonic structure reduces the fabricationdifficultness and the cost.

There have been many reported advances in the area of infraredphotodetectors. For example, Vasinajindakaw, et al. (2011) [12],discloses a quantum dot photodetector enhanced by Fano-type interferencein a metallic two-dimensional (2D) subwavelength hole array (2DSHA). Thephotocurrent enhancement wavelength shows an offset from the plasmonicresonant peak and corresponds to a dip in the transmission spectrum ofthe 2DSHA structure. The offset is attributed to the Fano-typeinterference in the 2DSHA structure. The asymmetric line shapes of theplasmonic resonance are analyzed and agree well with the two-peakFano-type interference model. Over 100% enhancement in photodetectivityand photoresponsivity is achieved at the wavelength of the Fano dip ofthe first order plasmonic mode. This reference does not teach the samephotodetector array as described in one embodiment of the currentinvention, particularly not including the surface passivation.

Another reference, Rogalski (2012) [13], discloses a wide review ofadvancements in the field of focal plane array technologies. The generaldesign of the QDIP portion of the focal plane array is described, but isnot backside configured. Additionally, the reference teaches thatsurface passivation can be problematic and that typically used materialsinclude: silicon nitride, silicon oxides, ammonium sulphide, andaluminium gallium antimonide alloys. This reference does not describebackside configuration of the QDIP or the inclusion of surface plasmonstructures layered above the contact layer and QDIP layers described inone embodiment of the current invention.

Another reference describes that the SiNx passivation layer helpsimprove the performance of QDIP (Vasinajindakaw (2009) [14]). However,this reference does not describe backside configuration of the QDIP orthe inclusion of surface plasmon structures layered above the contactlayer and QDIP layers described in one embodiment of the currentinvention.

Another reference discloses a quantum dots-in-a-well (DWELL) focal planearray with surface plasmon structures (Krishna (2011) [15]). Thereference also mentions that 200 nm thick Si₃N₄ was deposited forsurface passivation using plasma enhanced chemical vapor deposition(PECVD). Surface plasmon structures were also included in the array.However the positioning of the surface plasmon structures is not thesame as described in one embodiment of the current invention. Thisreference also does not describe backside configuration of the QDIP orthe inclusion of surface plasmon structures layered above the contactlayer and QDIP layers described in the current invention. The referencedoes not describe the inclusion in any focal plane array structure.

Another reference discloses a quantum-well infrared photodetector focalplane array (Eker, (2008) [16]), not a backside configured of the QDIPor the inclusion of surface plasmon structures layered above the contactlayer and QDIP layers described in one embodiment of the currentinvention.

Another reference discloses various advances in the field of longwavelength infrared quantum dot photodetectors and infrared quantum dotphotodetector focal plane arrays (Li (2010) [17]). The referencedescribes various passivation techniques (including Si₃N₄) are used toreduce dark current effects. This reference also does not describebackside configuration of the QDIP or the inclusion of surface plasmonstructures layered above the contact layer and QDIP layers described inone embodiment of the current invention.

Another reference discloses a focal plane array with plasmonicenhancement of a quantum-dot infrared photodetector (QDIP) using acorrugated metal surface (CMS) (Lee (2011) [18]). Passivation isdescribed as an Au (gold) film, rather than SiN_(x) passivation. Thisreference also does not describe backside configuration of the QDIP orthe inclusion of surface plasmon structures layered above the contactlayer and QDIP layers described in one embodiment of the currentinvention.

Another reference discloses a quantum dot infrared photodetector focalplane array, U.S. Pat. No. 6,906,326 [19]. Elements not discussed arethe inclusion of surface plasmons and the inclusion of a passivationlayer. This reference also does not describe backside configuration ofthe QDIP or the inclusion of surface plasmon structures layered abovethe contact layer and QDIP layers described in one embodiment of thecurrent invention.

Another reference discloses that plasmonic enhancement of a quantum dotinfrared photodetector (QUIP) integrated with a metal photonic crystal(MPC) depends on the direction of the incident light, air-side versussubstrate-side illumination (Lee et al. (2010) [20]). Compared withair-side illumination, substrate-side illumination on the samephotodetector results in more than two times enhancement in detectivity.This is explained by more efficient excitation of surface plasma waves(SPWs) at the MPC/QDIP interface in the back-side geometry. Theair/MPC/semiconductor structure is optically asymmetric and hasdifferent SPW coupling leading to higher photoresponse forsubstrate-side illumination. This agrees with simulation and providesdirect evidence that the detectivity enhancement is due to the couplingto SPWs and is crucially affected by light incident direction. Thereference further describes that for better comparison, substrateremoval for closer proximity between an air-side and a substrate-sideaperture, and metal passivation over extra openings for equal irradiancein both air-side illumination (ASI) and substrate-side illumination(SSI) are required. This reference also does not describe the inclusionof surface plasmon structures layered above the contact layer and QDIPlayers described in one embodiment of the current invention. Thereference does not include the passivation layer. The reference alsodoes not describe the inclusion in focal plane arrays.

ADDITIONAL BACKGROUND OF THE INVENTION

A photodetector is a device that converts the electromagnetic field,such as visible light or infrared light, into electrical signal. In theinfrared (IR) regime, photodetectors can be categorized into two groups.The first group includes thermal detectors that detect heat. One suchthermal detector, which based on thermal conductor effect, is called abolometer. A bolometer is a device for measuring the power of incidentelectromagnetic radiation via the heating of a material with atemperature-dependent electrical resistance. Another, thermal detector,which based on thermoelectric effect, is called thermopile. A thermopileis an electronic device that converts thermal energy into electricalenergy. It is composed of several thermocouples connected usually inseries or, less commonly, in parallel. The second group includes photodetectors that detect electromagnetic radiation. The detection is basedon the principle of quantum mechanics. Photo detectors can becategorized into two types depend on the transition of electron on theband structure, one is an interband transition photodetector and theother is an intersubband transition photodetector.

The interband transition photodetector employs the transition ofelectromagnetic radiation, which creates excess electron-hole pair inthe conduction band and valance band. The current state of the art ofinterband transition photodetector is mercury cadmium telluride (MCT,HgCdTe). By using growth techniques such as liquid phase epitaxy (LPE)or molecular beam epitaxy (MBE), the composition can be changed in theform of Hg_(1-x)Cd_(x)Te. Spectral range of Hg_(1-x)Cd_(x)Te can bedesigned to cover in SWIR range, MWIR range, or LWIR range. MCT has fastresponse time and high sensitivity compare to other IR detectors.However, the cost is relatively high and its noise current is high sothat its application of LWIR needs to be cooled down to temperature near77K to reduce noise in the device.

The intersubband transition photodetector employs only one band such asconduction band. By manipulating the band structure with precise controlof MBE deposition; one can create the intersubband transition ofelectron on the conduction band. Photodetector based on intersubbandtransition are quantum well infrared photodetector (QWIP) and QuantumDot photodetector (QDIP).

QWIP have several advantages over other IR detector. First, a QWIP haslow cost and good uniformity compared to other IR detectors. Second, itoffers high sensitivity with wavelength flexibility in mid-wavelengthinfrared (MWIR) and long-wavelength infrared (LWIR) regions [21]. Third,it has multicolor capability over the other IR detectors [21]. However,QWIP still have the disadvantage due to its inability to absorb normalincident radiation [22].

QDIP was developed from QWIP. The basic principle operation of QDIP isquite similar to QWIP. QDIP offers several advantages. QDIP also hasmulticolor capability. QDIP has three dimensional quantum confinementswhich enable sensitivity on normal incident radiation [11]. Moreover,QDIP also has a lower dark current than a QWIP since the density ofstates in quantum dot are much lower than that in quantum well, whichallows QD to hold less thermally-generated electrons [23]. In addition,QDIP has long excited state lifetime which allows efficient collectionof photo-excited carriers and this leads to high photoconductive gain[23]. However, QDIP detectivity is still lower than that of QWIP. Thisleads to further inconvenience which appears that QDIP has to operate atlow temperature. Therefore, there is need for QDIP to increase itsdetectivity to match and exceed the performance of current IR detectors.

DETAILED DESCRIPTION OF THE INVENTION

The standard FPA fabrication starts from MBE growth wafer as in FIG. 1.The brief fabrication process is explained here. In one embodiment, aMBE growth wafer is fabricated into FPA by standard photo-lithography,wet etching procedures, E-beam metal evaporation deposition, lift-offprocess, thermal annealing, and Indium bump deposition. FIG. 1 shows thesemi-insulating GaAs substrate 1 upon which the first contacting layer 2is deposited. Upon the first contacting layer 2 a first buffer layer 3is also deposited. Directly above the first buffer layer 3, the QDIPlayer 4 resides. Above the QDIP layer 4 is the second buffer layer 5.Above the second buffer layer 5 is the second contacting layer 6. FIG. 2shows the finished structure of a regular QDIP. The finished structureincludes the processed MBE growth wafer from FIG. 1 after wet etchingprocessing which further includes a first set of metal contacts 7attached to said first contacting layer 2, and the second metal contact8 deposited upon said second contacting layer 6. Above the second metalcontact 8 is an indium bump 9. In one embodiment, a wafer may thenhybridized with a read out integration circuit (ROIC) 14 for imagesignal processing. IR light may shine from the bottom side (referred toas a backside illumination) to the detector since said ROIC 14 may beopaque.

Recently, surface plasmon polaritons (SPP) on thin film metallic holearray 13 has been studied extensively because of its ability to enhancethe near-field of electromagnetic wave that pass through it. With acareful design of geometry on the metallic hole array and the correctmetal type, one may couple SPP into the surface between metal andsurrounding media. One can also control SPP properties such aswavelength that propagate along surface and its bandwidth. Research hasdemonstrated an enhancement of photocurrent and photo responsivity onSPP QDIP on single mesa [12]. Over 100% enhancement is achieved atspecific wavelength, 8.5 μm. Other researchers also report enhancementon SPP QDIP [18, 24].

In one embodiment, FIG. 3 shows one embodiment of the current inventionQDIP structure. In one embodiment, a purpose of the design is to add aSPP structure 10 on top of QDIP structure 4 to create a SPP wave alongthe metal/dielectric interface. In one embodiment, a passivation layer(11 and 12) is deposited on top of SPP structure 11 and FPA sidewall 12for two advantages. First advantage is to separate SPP structure 10 fromIndium bump 9, which allows SPP wave on both sides of metal. Thepresence of a passivation layer (in one embodiment, Silicon Nitride,SiN_(x)) may help to maximize the resonance of SPP 10 thus increaseenhancement further [25]. The second advantage is to reduce the surfacedark current on FPA [14]. In one embodiment, FIG. 4 shows a physical SPPstructure 10, a metallic hole array is made, in one embodiment, by goldwith 75 nm thicknesses. The hole diameter may be 1.3 μm and the periodmay be 2.6 μm.

The preliminary results show a strong enhancement of photocurrent onFourier Transform Infrared Spectroscopy (FTIR) test in all five biasvoltages. The results show two strong enhancement peaks, at 8.5 μm and5.9 μm, and may indicate the multispectral enhancement. Following isbrief description of the report. First, sixteen single mesas arefabricated by standard photo-lithography. The mesa shape is cylindricalwith 250 μm diameter. Then SPP structure are deposited on eight singlemesas called SP mesa, leave the other eight bare mesa called bare mesa.Finally, the SP mesas and bare mesas are tested simultaneously tocompare the performance.

There are several novel aspects of the invention and improvements uponprior art. The presence of a SPP structure may increase photocurrent ona detector. Therefore, it may improve detector performance. The designof SPP structure may also offer multispectral sensing application. Thepassivation layer may decrease surface dark current on the sidewall ofthe pixels of the FPA. Reducing dark current may increase thedetectivity thus improving an overall performance of the detector. Thebackside configured plasmonic structure may significantly reduce thedifficultness to make an image array and thus greatly reduces the cost.

DEVELOPMENT OF THE INVENTION

Herein an innovative design of quantum dot infrared photodetector (QDIP)for focal plane array with the assistance of surface plasmon structureand silicon nitride (SiN_(x)) passivation layer is presented. In oneembodiment, QDIP performance as measured by a photocurrent spectrumresponse is enhanced by sixteen times. Both analytical expression andexperimental result support the existence of surface plasmon polaritons.Even though the experimental data are based on QD infrared detectors, inone embodiment, the invention can be used for any kind of detectors andFPAs for performance enhancement. The innovation design for focal planearray is discussed in greater detail below.

Quantum dot infrared photodetectors (QDIP) have been developed over thelast twenty years. The infrared detection and imaging in the long waveinfrared (LWIR) have been used in many applications such as missiletracking, night vision imaging and thermal sensing [21, 22, 26, 27]. Inorder to compete with state of the art, which is Mercury CadmiumTelluride detector (MCT); QDIP needed to improve its overallperformance. There may be two possible solutions, the first is toincrease photocurrent and the second is to decrease dark current.

Surface plasmon polaritons have drawn the attention of scientists andresearchers in the past few years [28-36], including, but not limitedto, detector and sensor fields [37-45]. Surface plasmon structures havebeen applied onto QDIP to enhance the photocurrent. The strongelectromagnetic field on metal/dielectric interface induces the photonabsorption which leads to increasing photocurrent [24, 46-48]. However,the numbers of reports are still limited and the performance enhancementhas yet to be improved. Recently, a monolithically integrated plasmonicinfrared quantum dot camera had also demonstrated [49]. The surfaceplasmon structure was integrated on the back side of substrate byinterferometric lithography technique. However, fabrication technique israther complicated and expensive for industrial production. Theperformance enhancement, as well, has yet to be improved.

In one embodiment, a back-side configured surface plasmon enhancedquantum dot infrared photodetector with a surface plasmon structure andSiN_(x) passivation is used in order to increase the photocurrent anddecrease the dark current. Standard photolithography is usually used forthe fabrication process, which eliminates the complication offabrication process and reduces fabrication cost together withprocessing time.

Sample Growth and Device Fabrication

The QDIP (UML273) may be grown by molecular beam epitaxy (MBE) using aV80H MBE system. A 0.3 μm Si-doped(n+) GaAs contact layer (n=1×10¹⁸cm⁻³) may be first grown on a semi-insulating GaAs (100) wafer. Thegrowth temperature for the GaAs contact and buffer layers is 620° C. Theactive absorption region includes, but is not limited to, ten periods ofQD heterostructures consists of 1 nm In_(0.15)Ga_(0.85)As followed by2.0 monolayer (ML) of InAs, 30 ML of In_(0.20)Ga_(0.80)As cap layer, and50 nm GaAs spacer layer. The growth rates of the InAs QDs,In_(0.20)Ga_(0.80)As cap layers, and GaAs spacers were 0.16, 0.8, and0.9 ML/s, respectively. The doping level of the QD region was estimatedto be 3.5×10¹⁷ cm⁻³. The QD layers and the In_(0.20) Ga_(0.80)As caplayers were grown at 470° C. The top contact layer may be highlySi-doped (n=1×10¹⁸ cm⁻³) GaAs with a thickness of 0.1 μm. FIG. 5 showsthe structure of the UML273 sample.

After the growth of the layers described above, the wafer was processedinto 250 μm diameter circular mesas using standard photolithography andwet etching procedures. The top and bottom electrodes were formedsimultaneously on top and surrounding of the mesas by standard e-beammetal evaporation deposition, lift-off, and thermal annealing processes.The QDIP was then wire-bonded and mounted on a cold finger inside atemperature-controllable infrared (IR) dewar with a ZnSe IR window. Thephotocurrent spectrum of the QDIP was measured using a Bruker OpticsTensor27 FTIR spectrometer.

Two-Dimensional (2D) Sub Wavelength Hole Array (2DSHA) on QDIP

Afterward, surface plasmon structure of 2.6-1.3 μm on top surface of theQDIP was fabricated with standard lithography. The metal thickness wasoptimized for the performance enhancement. In one embodiment, it wasfound that for the surface plasmon structure, the gold thickness of 75nm have the maximum performance [50]. In one embodiment, each of theplasmonics sample has bare detector that stands side by side used forreference purpose. Both plasmonics sample and reference sample aretested in controlled environment and compared for the enhancement. FIG.6, FIG. 7, and FIG. 8 are the SEM pictures of plasmonics structure onQDIP.

SiN_(x) Deposition and Etching on QDIP

There are several types of passivation layers, such as Si₃N₄, SiO₂ andpolyimide (PI), that may benefit electrical devices. The potentialbenefits of each of these passivation layer types have been compared andreported. It has been shown, both theoretically and experimentally, thatSi₃N₄ with a thick thickness may be most suitable to use as apassivation layer compared to SiO₂ on InP for multi-quantum wellFabry-Perot laser structure [51]. One of the reports also indicate thatSi₃N₄ and PI outperform SiO₂ on reducing the leakage current onphotodiodes [52]. On an Avalanche photodiode based on GaAs, Si₃N₄passivation was more effective than a PI passivation in restricting thesurface current. Si₃N₄ passivation has a lower Generation-Recombination(G-R) current and a surface carrier concentration than PI passivation[53, 54]. On the bipolar transistors, Si₃N₄ passivation also improvedthe reduction of surface recombination [55, 56]. On GaAs metalsemiconductor field effect transistors (MESFETs), Si₃N₄ passivationproved to have a better reliability compared to SiO₂ passivation [57].Another advantage of Si₃N₄ passivation on the device is to preventenvironmentally induced degradation and surface current reduction. Mostof the reports used a plasma-enhanced chemical vapor deposition (PECVD)system to deposit SiN_(x) passivation layer on the material.

In one embodiment of the current invention, the SiN_(x) passivationlayer was applied on top and sidewall of the sample for two possibleadvantages. The first advantage may be to separate surface plasmonstructure from Indium bump which allows surface plasmon wave on bothsides of metal. The passivation layer helps to maximize the resonance ofsurface plasmon wave thus increase enhancement further [25]. Compared tothe back side surface plasmon structure [49], the presence of surfaceplasmon structure on top of the mesa may also reduce the crossinterference of surface plasmon wave between adjacent mesa since thesurface plasmon structure was separated apart in each mesa. The secondadvantage may be to reduce the surface dark current on FPA side wall[14]. FIG. 2 shows a standard FPA design for the commercial applicationand FIG. 3 shows one embodiment of the current invention, a pixel 15 ofthe FPA design.

Before SiN_(x) deposition, the QDIP sample was polished on the backsidein order to be tested for backside illumination. The backside polishingon reference GaAs wafer has been tested and it was found thetransmission through the wafer is consistent over 94% transmission incomparison with the commercial double side polishing GaAs sample(mirror-finished grade). SiN_(x) was then deposited on the wafer byplasma-enhanced chemical vapor deposition (PECVD) machine (Nexx PECVD,Cirrus 150) with the thickness of 0.4 μm. One embodiment of thedeposition recipe is found in Table 1 (PECVD Si₃N₄ Deposition Recipe).After deposition, the wafer was spin coated with photoresist and softbaked. The metal-deposition mask may be used again to align the waferand the mask may be exposed so the photoresist had an opening area ofthe metal structure. Next, reactive ion etching (Nexx RIE, Cirrus 150)was applied to etch SiN_(x) down to reach the metal layer. Oneembodiment of the RIE Etching recipe is on Table 2 (RIE Si₃N₄ EtchingRecipe).

TABLE 1 PECVD Si₃N₄ Deposition Recipe Ar (sccm) 20 N₂ (sccm) 11.6 3%SiH₄ (sccm) 110 Process Pressure 20 (mTorr) Microwave Power (W) 265 RFPower (W) 0 He Backside Cooling 5 (Torr) Deposition Time (sec) 1600Deposition Rate (A°/sec) 2.57

TABLE 2 RIE Si₃N₄ Etching Recipe. Ar (sccm) 10 CF₄ (sccm) 15 ProcessPressure 10 (mTorr) Microwave Power (W) 300 RF Power (W) 100 He BacksideCooling 5 (Torr) Process Time (sec) 145

Device Performance Characterization and Discussion

For surface plasmon polaritons on a metallic thin film, the wavelengthsof the excited surface plasmon modes at normal incidence can beexpressed as in Equation 1 found below [31].

$\begin{matrix}{\lambda_{ij} = {\frac{a_{0}}{\sqrt{i^{2} + j^{2}}}\sqrt{\frac{ɛ_{m}ɛ_{d}}{ɛ_{m} + ɛ_{d}}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where a₀ is the period of periodic hole array on metal thin film, i andj are integers, ∈_(m) is dielectric constant of metal, ∈_(d) isdielectric constant of surrounding medium.

One can achieve the wavelength of surface plasmon polaritons bydesigning the period of holes array on metal thin film. In the visibleregime, several experimental and simulation results have been reportedon the enhancement of transmission on holes array metallic thin film[25, 58-60]. In infrared regime, the formula on Equation 1 is stillapplicable with a substantial change on ∈_(m).

The surface plasmon structure was first fabricated on a commercialdouble side polished bare GaAs wafer and tested the transmission profileon each period. FIG. 9 shows the transmission profile of surface plasmonstructure for different period. The plasmonic structures are fabricatedby standard lithography on GaAs wafer with gold thickness of 25 nm. Theresults show the transmission profile of two-dimensional subwavelengthhole array (2DSHA) from the period of 2.2 μm up to 3.0 μm. On the labelin FIG. 9, the front number indicates period and the following numberindicates circular hole size diameter. The physical geometry of 2DSHA issimilar as FIG. 6, FIG. 7, and FIG. 8, except the 2DSHA is on doubleside polished bare GaAs wafer. All samples have 50% filling factor.

Each transmission profile on FIG. 9 displays two obvious peakscorresponding to surface plasmon modes in Equation 1. As an example,∈_(d) is equal to 10.89 for GaAs. ∈_(m) is approximately equal to−5050+1090i for gold at wavelength of 10 μm [61]. For the calculatedsurface plasmon mode with the 3.0 μm period, λ₀₁=λ₁₀=9.91 μm, λ₁₁=7.0μm. For the period of 2.6 μm, ∈_(m) is approximately equal to−2507.5+i1059.9 for gold at wavelength of 8.8 μm and ∈_(d) is equal to10.89 for GaAs. The calculated wavelength are λ₀₁=λ₁₀=8.60 μm, λ₁₁=6.08μm.

Herein, the period of the surface plasmon structure at 2.6 μm wasselected to match the QDIP spectrum response. The gold thickness onsurface plasmon structure was optimized and it was found that themaximum photocurrent corresponds to the metal thickness of 75 nm [50].In FIG. 10, the black color lines show the spectrum response of thecurrent QDIP sample with Indium bump deposition on top of the mesa. TheQDIP sample was polished on the backside and illuminated from thebackside. The spectrum response for the current QDIP sample is from 4 μmto 10 μm. The peak of transmission profile on FIG. 9 and the analyticalexpression on Equation 1 confirm the essence of surface plasmongeneration on the surface of metal/dielectric medium.

FIG. 10 exhibits the spectrum response of QDIP-SP sample and QDIP-refsample (QDIP-reference sample). The QDIP-SP (red color lines) indicate asignificant enhancement in every applied bias voltage compared withQDIP-ref (black color lines). The spectrum response of four biases,V=−0.5V, V=−0.7V, V=−1.3V, and V=−1.8V was demonstrated. The spectrumresponse ranges from 4 μm to 10 μm in all biases. The low voltage biashas a tendency to pick up stronger in shorter wavelengths (3-7 μm) dueto quantum-confined Stark effect. The QDIP-SP sample has two peaks onspectrum response, one is at 8.8 μm and the other is 6.1 μm. At thevoltage bias V=−0.5V, the QDIP-ref spectrum response is roughly in thesame level. But QDIP-SP spectrum response has stronger first peak thanthe second peak. This means that the surface plasmon wave generationefficiency for the first mode may be higher than that of second mode.FIG. 11 also suggests that first mode efficiency may be higher than thesecond mode efficiency. The surface plasmon generation efficiency can berelated to the imaginary part of the surface plasmon polaritons wavevector (k_(spp)). The greater imaginary part, the more internal dampingthus more loss it will be. For wavelength 6.1 μm, ∈_(m) is approximatelyequal to −1320+382i. For wavelength 8.8 μm, ∈_(m) is approximately equalto −2420+1030i. Surface plasmon momentum wave vector (k_(spp)) isexpressed in Equation 2 found below [62, 63]. Providing

$\begin{matrix}{k_{SPP} = {k_{0}\sqrt{\frac{ɛ_{m}*ɛ_{d}}{ɛ_{m} + ɛ_{d}}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

one can calculate k_(spp). The result is the following,k_(spp-6.1μm)=k₀(3.3264+0.0037i) and k_(spp-8.8μm)=k₀(3.3200+0.0027i).The imaginary part for 8.8 μm is less than the one for 6.1 μm, whichsuggests less loss on the surface plasmon polaritons wave and thereforeleads to greater performance enhancement on the spectrum response.

${k_{o} = \frac{w}{c}},$

FIG. 11 shows the enhancement ratio of spectrum response between QDIP-SPand QDIP-ref. Four bias voltages were tested and it was found theenhancement ratio are consistent in all bias voltages from V=−0.5V toV=−1.8V. There are two peaks of enhancement. The first peak ofenhancement ratio is located at 8.8 μm wavelength which corresponds tomode 01 (λ₀₁=8.60 μm) of the surface plasmon generation on themetal/dielectric surface. The second peak of enhancement ratio is alsocorresponds to mode 11 (λ₁₁=6.08 μm). The significant enhancement ratioat sixteen times is reported at 8.6 μm wavelength for mode 01. Theenhancement ratio at four times is also reported at 6.1 μm wavelengthfor mode 11. Note here that two wavelengths of enhancement depend on theperiod of the surface plasmon structure as stated in Equation 1.Multicolor enhancement selection has been reported depending onpolarization selection by a designed surface plasmon structure [64].

In addition to the enhancement result of sixteen times and four times at8.6 μm and 6.1 μm, it is believed that the surface plasmon structure ontop of mesa and SiN_(x) passivation layer between surface plasmonstructure and Indium deposition offers several advantages on theinfrared focal plane array and imaging array. First, all fabrication arecarried out on the top side of the sample. This will reduce timeconsumed in the fabrication process compared to fabrication of surfaceplasmon structure on the back side of the sample. Moreover, the distancebetween the top side surface plasmon structure and quantum dotabsorption layer is closer than the back side's one due to thelimitation of the polishing process and the standard design of thickern⁺ GaAs contact layer on the bottom electrode. Therefore, the effect ofsurface plasmon generation is exponentially stronger with a top sidesurface plasmon structure than a back side surface plasmon structure.The second advantage is related to the cross talk of surface plasmonwave on the metal/dielectric interface. The location of each mesa onfocal plane array may be very close to each other, in fact approximatelya few microns in distance. A surface plasmon wave from the backsidestructure generated on one mesa can travel along the metal/dielectricinterface and affect the adjacent mesas as long as the metal/dielectricinterface is connected. As a result, the cross talk signal is introducedon the back side surface plasmon structure. The smaller distance fromone mesa to each other, the stronger cross talk signal interferes withthe correct signal. For the current invention design, the surfaceplasmon structure on top of each mesa is isolated from all adjacentmesas. So, the cross talk from surface plasmon wave will not begenerated. The quality of the picture would be better compared with thebackside surface plasmon structure. The third advantages may come fromthe SiN_(x) passivation layer. A SiN_(x) passivation layer will reduceoverall dark current since it reduces the surface dark current on theside wall. The effect of dark current reduction may be signified withthe smaller mesa size on focal plane array due to the geometry of thesurface and volume on the mesa. The reduction of dark current fromSiN_(x) passivation layer may benefit the detectivity of the detectorand hence improves its performance. Indium bump on top of SiN_(x)passivation layer may also reflect the light back to the sample, whichmay lead to stronger surface plasmon wave generation. With thecombination of all three advantages from the invention's current designand the maximum enhancement ratio of sixteen times, the development ofimaging array and infrared photodetector has been improved to offergreater performance and better image quality.

The performance enhancement may also be improved by reducing theseparation distance between the quantum dot absorption layer and surfaceplasmon structure. With the top side surface plasmon structure design,it may be more convenient to thinner the top n⁺ GaAs contact layer dueto non-etching process on the top electrode. GaAs spacer layer andbuffer layer could also be re-designed to reduce the separationdistance.

In summary, a QDIP with surface plasmon structure with significantenhancement of sixteen times at particular wavelength has beendemonstrated. The wavelength of enhancement may be designed by geometryof surface plasmon structure to match QDIP spectrum response. It beganwith the transmission profile of surface plasmon structure on GaAs. Thensurface plasmon structure on the QDIP was integrated. With an assistanceof SiN_(x) passivation layer in between surface plasmon structure andIndium bump on top of the detector, the performance enhancement ofsixteen times on the first mode and four times on second mode ofplasmonic wave generation as achieved.

More importantly, the innovative QDIP-FPA with surface plasmon structurefor optimizing the imaging array performance has been outlined. Theresult in single mesa may be a validation of the new FPA approach. ThisFPA design resolves a complexity of fabrication process in QDIP-FPA withsurface plasmon structure, offers a cost and time reduction on FPAfabrication process, and improve the detector performance by applyingsurface plasmon structure in conjunction with SiN_(x) passivation. ThisFPA design together with the enhancement result may significantly impactthe manufacturing of infrared photodetectors.

Although the invention has been described with reference to thesepreferred embodiments, other embodiments can achieve the same results.Variations and modifications of the present invention will be obvious tothose skilled in the art and it is intended to cover in the appendedclaims all such modifications and equivalents. The entire disclosures ofall applications, patents, and publications cited above, and of thecorresponding application are hereby incorporated by reference.

One Example of Fabrication Steps

-   -   1. Form the two dimensional hole array (2DHA) plasmonic        structure using photolithography upon the layered QDIP        structure, then deposit the metal (optimally 75 nm gold), and        lift-off to finish the patterning.    -   2. Pattern the mesa using photolithography, then wet etch using        a piranha etch (sulfuric acid, hydrogen peroxide, water),        stopping the etch at the bottom contact layer.    -   3. Pattern the ohmic contacts, on the top of the pixels and the        ground ring, using photolithography, then deposit Ni(50        Å)/Ge(170 Å)/Au(330 Å)/Ni(150 Å)/Au(3000 Å) and lift off to        finish the ohmic contacts.    -   4. Use rapid thermal annealing in a nitrogen environment to form        the ohmic contacts.    -   5. Deposit 400 nm of low stress Si₃N₄ for passivation and to        protect the 2DHA from the indium.    -   6. Open a via to the ohmic contact metal using an inductively        coupled plasma reactive ion etch (ICPRIE) system.    -   7. Use photolithography to pattern the indium bumps, then        deposit Cr (150 Å)/In (5 μm) and lift-off to finish the indium        bump step.    -   8. Hybridize to read out integration circuit (ROIC).        An example of a product produced by such a process is seen in        FIG. 14.

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1-13. (canceled)
 14. A method of producing a focal plane arraycomprising, a) formation of a two dimensional hole array plasmonicstructure upon a QDIP structure using photolithography, followed bydeposition of metal to form a mesa, b) patterning of said mesa usingphotolithography, followed by wet etch stopping at the bottom contactlayer of said QDIP structure, c) formation of ohmic contacts, d)deposition of a passivation layer, e) opening of a connection to theohmic contact through passivation layer, f) patterning and deposition ofan indium bump, and g) hybridization of said indium bumps to a read outintegration circuit.
 15. The method of claim 14, wherein said metaldeposited to form a mesa comprises gold.
 16. The method of claim 14,wherein said mesa is between 75 and 30 nm.
 17. The method of claim 14,wherein said mesa is circular.
 18. The method of claim 14, wherein stepc) comprises: i) masking said QDIP structure; and ii) patterning saidohmic contacts using photolithography, followed by deposition of metaland lift off of said to finish the ohmic contacts.
 19. The method ofclaim 18, wherein said method further comprises rapid thermal annealing.20. The method of claim 18, wherein said metal comprises Ni/Ge/Au/Ni/Au.21. The method of claim 20, wherein said metal thicknesses compriseNi(50 {acute over (Å)})/Ge(170 {acute over (Å)})/Au(330 {acute over(Å)})Ni(150 {acute over (Å)})/Au(3000 {acute over (Å)}).
 22. The methodof claim 14, wherein step d) further comprises deposit low stress Si₃N₄.23. The method of claim 14, wherein step e) comprises opening to theohmic contact metal using an inductively coupled plasma reactive ionetch system.
 24. The method of claim 14, wherein step f) comprisesdeposition of Cr/In and lift-off.
 25. The method of claim 24, whereinsaid deposition further comprises deposition of 150 {acute over (Å)} Crand 5 μm In.